Organofunctional silanes are a class of dual-functional chemicals that are characterized by their ability to react with mineral surfaces like glass, silica, silicates, alumina, metallic surfaces and more generally, surfaces with strong polarity. They can also react with or compatibilize organic materials like plastics, rubber, coatings and resins. Such silanes are used in filled and reinforced materials where their ability to react with both the filler or reinforcement and the organic matrix provides enhanced adhesion and improved physical properties. In such uses they are often called "coupling agents". Most such silanes are liquids.
Silane masterbatches (solid silane concentrates, i.e., dry or powder from silane) have been used for many years in applications where it is desirable to add the silane as a solid. The masterbatches are usually based on a porous solid carrier with high adsorption capacity and carry up to 75% liquid silane. For economic reasons, it is desirable to adsorb as much silane as possible because the carrier usually has no other function than to carry the silane, and then release it essentially completely into the compound during processing, and is selected for its absence of reactivity with the polymers and with the silanes (which reactivity could cause the silane to be destroyed or consumed by reactions induced by the carrier).
Numerous carriers have been used or tested for such purposes, including porous organic polymers, synthetic silicates, diatomaceous earth, silica, carbon black, and the like. The carriers differ by their particle size, structure and composition, and therefore by their adsorption ability. The finest grades of carriers, i.e., with a particle size of less than 1 micron, may also participate in the composite material properties as fillers or reinforcing components. However, it has been recognized that not all fillers are equivalent, not only in terms of reinforcing and adsorption power, but also in their inertness towards silanes. An attempt to use silica as the carrier produced a product with very restricted shelf life and poor storage stability. It has been demonstrated that some carriers, including silicas, react with silanes; the reaction consists of a hydrolysis due to unavoidable moisture traces on the carrier surface, and subsequent condensation leading to a large proportion of polysiloxane and/or silane bound irreversibly to the carrier surface. The condensation reaction is particularly detrimental to the silane coupling efficiency, because it strongly immobilizes the silane on the carrier surface and prevents its ultimate release, dispersion and usefulness in the filled or reinforced compounds. Silane polymerization on the carrier due to carrier surface reactivity and presence of moisture usually results in poor physical performance of the resulting compound. More important, some or all of the expensive silane is wasted, that is, it cannot disperse during compounding and transfer to other filler particles as intended. The amount of polymerized silane varies with moisture content of the carrier before silane loading, with time and temperature, and especially with the filler surface activity. These parameters are very difficult to control and maintain within narrow specifications. If a carrier could be inert toward premature destruction of the silane, yet be reactive with the silane under compounding or processing conditions, it might serve the dual functions of inert carrier and reactive reinforcing filler. These two goals are in conflict.
To solve these problems, it has been suggested to pre-dry the carrier surface and/or to coat it with a low-cost or non-reactive silane before loading the active silane to avoid any water-induced reaction. However, these processes are not economically feasible and are not favored. Furthermore, it is difficult to eliminate all water from the carrier surface without impairing its adsorption properties. This is usually due to the structure of mineral carrier surfaces, which generally consists of layers of strongly adsorbed water. The physisorbed water on the silica can be eliminated by heating the silica to a temperature of about 105.degree. C. It can be re-absorbed by exposure to moist air. The chemisorbed water is very strongly held through hydrogen bonds and is eliminated at about 200-250.degree. C. This water is close to ionic sites on the carrier surface, and its properties are highly dependent on surface impurities of the carrier surface itself. For instance, it is well known that activating a mineral at temperatures of about 250.degree. C. can make it more reactive towards organic reactions like condensation and radical polymerization. This property has been widely used in catalytic processes. Therefore, dehydrating a mineral at such temperatures often does not reduce its reactivity towards silanes. Constitution water is formed by condensation of pairs of neighboring hydroxyl groups, especially those hydroxyl groups which are very strongly hydrogen-bonded silanols, to liberate water at temperatures between 250 and 800.degree. C. Therefore, to totally eliminate all reactive groups from the carrier surface, it is necessary to calcine the mineral at elevated temperatures. Unfortunately, most carriers will not retain their structure at such temperatures and therefore, this level of drying is not practical.
The availability of carried silane for participation and dispersion in compounding and mixing may be measured by a solvent extraction procedure. A solvent which easily dissolves unreacted silane is used. If the silane can not be extracted, it is also unavailable for compounding and coupling.
For many filled rubber and polymer systems, in which the silane is used to create a covalent bond between the polymer and the filler surface, it is also desirable to use a white silane masterbatch to produce white or lightly colored compounds. White carriers include silicas, silicates, diatomaceous earth, alumina and more generally non-metal oxides. Some of these carriers provide added benefits because they can also behave as fillers in the compound. However, their surface structure is highly polar and tends to contain significant amounts of strongly absorbed water. Furthermore, surface metal ions that generally are present on such carriers can catalyze the silane hydrolysis and condensation reactions. These reactions proceed with varying speeds at room temperature, and are the cause of aging of silica-carried silanes, so that a limited shelf-life has to be imposed onto such masterbatches. Therefore the rubber industry, and more particularly the tire industry, has largely selected carbon black carriers wherever black compounds were produced.
Non-silica carriers, particularly carbon black or porous polymers, are less reactive and contain low levels of weakly absorbed water. However, they are usually more expensive than minerals. Carbon black cannot be used in most colored compounds, though for black rubber applications, like automotive tires, it is possible to use carbon black as a carrier for silanes such as polysulfide silanes.
Now it has been found surprisingly that a specific class of silica fillers are inert during months of shelf storage at e.g. room temperature towards destructive silane condensation reactions, which prevent the silanes from being desorbed when needed, and that their reactivity in this regard is even lower than carbon black carriers used currently in the rubber industry. These silicas are characterized by a high adsorption ability, a low microporosity, a low differential in the infrared absorbance at 3502 cm.sup.-1 at 105.degree. C. and at 500.degree. C. as defined herein and in preferred embodiment a surface chemical structure that is modified by the presence of an oxide with acidic character. As an added benefit, these particular silicas may actually have the characteristics of a "reinforcing" filler, rather than a mere inert carrier. Thus the conflicting goals of both "non-reactivity" during shelf storage and eventual reinforcing capability through reactivity in the rubber compounding and curing steps can be met in the same non-carbon-black carrier material.